Numerous studies have demonstrated the regulatory capability of peptide/MHC determinant density on T lymphocytes. In particular, we have shown that the avidity of a CTL population can be controlled by the amount of antigen presented on the APC and that avidity is a property that significantly affects the ability of the CTL to clear virus in vivo. Little is known about the way in which avidity is controlled in CTL. The goal of these studies is to define the cell surface molecules involved in determination of CTL avidity. This goal will be sought by evaluating the TCR usage, the requirement for costimulation, and the helper dependence of high versus low avidity CTL. The role of the TCR in determining avidity will be assessed in high and low avidity CTL by precursor frequency and Vbeta/Valpha analysis and by analysis of non-TCR factors controlling avidity in CTL of various avidity generated using a common TCR. The efficiency of various APC populations to activate high versus low avidity CTL will be analyzed. These APC populations possess diverse costimulatory molecules that may be differentially required by high and low avidity CTL. To further investigate the requirement for costimulatory molecules, the dependence on CD28 and CTLA4 signals will be evaluated. Finally the requirement for exogenous cytokines provided by CD4+ helper cells will be determined by generation of high and low avidity CTL in vitro in the absence of added growth factors and in vivo by elicitation in the absence of CD4+ cells. At the conclusion of these studies, we should gain new insights in the ways in which peptide/MHC determinant density regulates CD8+ cytotoxic T lymphocytes. Specifically we should understand more fully the repertoire of precursors available for response to a particular antigenic peptide, the contribution of the TCR to the avidity of an individual CTL, and the effect of signals provided by costimulatory molecules and cytokines on the activation and proliferation of CTL of defined avidity. These findings may contribute to the development of improved immunotherapeutic and vaccination strategies in which the selective activation or deletion of CTL possessing a defined avidity would be beneficial.